Recently sound proof material in a broad sense is used in not to speak of automobiles, but a number of fields such as electric home appliances and construction materials. It plays an important role for reducing noise, which constitutes a great social problem.
Noise is produced generally in such a manner that various sorts of vibrations generated by movement in different kinds of machines and apparatuses originating it, are transferred to other members, are resonated therewith, and radiated them as noise. Vibration isolation materials are used for preventing this transfer of vibrations to other members (vibration isolation).
The vibration isolation materials described above are roughly classified into a group of coil springs and another group of vibration isolation rubbers. Coil springs are used for lowering vibration level by varying a natural frequency to avoid resonance with vibration having an object frequency, while vibration isolation rubbers attenuate vibration principally by internal friction within a visco-elastic body made of rubber to isolate vibration as the result.
Among the vibration isolation materials described above, generally vibration isolation rubbers are used for vibration isolation in a wide frequency range. In particular, a vibration isolation washer formed in a washer shape is used often in a portion bound by means of a bolt for the purpose of preventing that engine vibration in automobile, etc. is transferred to another member such as a head cover, an oil pan, etc. through a clamping bolt, etc. and radiated as noise.
Usual vibration isolation washers are mainly made of rubber-molded bodies and further in addition to them, various constructions such as a washer made of rubber alone, a washer in which metal plates are stuck by vulcanization adhesion on the upper and lower surfaces of a simple rubber body, etc. are used, depending on the field of utilization.
In particular, for the head cover of an engine a vibration isolation washer, in which a metal support is stuck to a surface of a rubber piece by vulcanization adhesion, is widely utilized. One of the most important features of this type of vibration isolation rubbers consists in that not only the sort and the shape thereof can be arbitrarily chosen, depending on the field of utilization, but also the vibration isolation capability can be freely designed. However this type of vibration washers have following drawbacks.
Capability of a rubber-molded vibration isolation washer with metal support depends significantly on properties which rubber has (hardness, tensile strength, compression and recovery, compression set, etc.) and further on heat resistant properties thereof as well as other resistances to environments, in order that it can exhibit stably vibration isolation and other required characteristics. Generally it is believed that these variations are unavoidable.
For this reason, although it can be used without any problem for a field of utilization where no great load is applied thereto, in the case where it is used in an environment where it is subjected to a thermal load as a vibration isolation washer for the head cover of an engine in an automobile, a bolt is loosen by creep of rubber and vibration isolation effect is lost. In addition, in the case where a gasket etc. for preventing outflow of engine oil etc. is clamped together, at the worst case sealing property is lowered so that liquid leaks out or the bolt falls off, which may cause a serious problem.
Apart therefrom, at clamping the bolt, since contact resistance between the rubber and a seat surface is great, the rubber is twisted or friction coefficient is increased and thus torque on the bolt cannot by controlled precisely. Therefore no required load (force) can be secured or the bolt is fastened with an unnecessarily great force in expectation thereof. As the result, the metal support is brought into contact with the seat surface, which gives rise to a problem that vibration isolation is lost, etc.
Further, since most washers have great exposed side surfaces of rubber in order to improve vibration isolation and deformation of the rubber is remarkable, problems may be produced that the height of the clamped bolt is unstable, that the rubber is crushed because of fastening with a too great force, that it is apt to be subjected to attack by different oils, salt water, etc., and so forth.
According to Utility Model JP-A-Sho 57-83254, a structure is conceived, in which a plurality of raw material sheets, in each of which a rubber visco-elastic layer is stuck to a metal plate formed in a dish shape, are laminated on each other, in order to improve the vibration isolation effect and a long term creep resistant property. However not only this conceived structure cannot exhibit the vibration isolation effect owing to prolongation and complication of the vibration transmission path stated later according to the present invention, but also since different laminated sheets move freely at fastening, unnecessary friction is produced at interfaces of rubber/rubber and rubber/metal, which gives rise to an inconvenience that the surface of the rubber is worn away or peeled.
Further, according to JP-A-Sho 60-81511, a structure is proposed, in which rubber layers having the vibration damping effect and two or three metal plates (or vibration damping alloy plates) are laminated and adhered alternately. However no measures against torsion of the rubber at clamping the bolt are found therein and problems such as peeling, break, etc. are pointed out.
Now problems on a pressed construction such as an oil pan, a ventilation buffle plate, etc. for an automobile will be described.
Heretofore, in view of giving a pressed construction such as an oil pan, a ventilation babble plate, etc. for an automobile a vibration damping property, it is known that a vibration damping member composed by putting a macromolecular visco-elastic layer made of rubber, resin, etc. between two metal plates in a sandwich shape is used as a pressed construction itself, which is formed by press, or a macromolecular visco-elastic layer made of rubber, synthetic resin, etc. is disposed on either one of the inner or the outer surface of the construction to obtain a vibration damping property.
Further, for the pressed construction described above, there are many cases where a sealing function is required together therewith in order to prevent oil leak, etc. from the construction. The former has an advantage that the sealing and the vibration damping function can be utilized together by using the sealing function of the macromolecular visco-elastic layer on the surface, while the latter requires a separate sealing member such as a gasket, etc.
It is absolutely necessary for the vibration isolation washer described above for the field of utilization where reliability is considered to be important as parts for an engine in an automobile to have not only a high vibration isolating property but also various characteristics such as resistance against creep, initial clamping properties (easiness to fasten, torque transmissibility, torsion strength), resistance against environment, etc. together and appearance of a vibration isolation washer fulfilling satisfactorily these conditions is desired.
On the other hand, in the pressed construction as described above such as an oil pan, a ventilation buffle plate for an automobile, a ventilation damping structure, in which a macromolecular visco-elastic layer is put between two metal plates (constrained damping material), has a disadvantage that cost is high in the aspect of fabrication, although it has a high loss factor. Further, in the case where the constrained vibration damping structure should have a sealing property, it requires a separate gasket.
On the contrary, for a vibration damping construction, in which a macromolecular visco-elastic layer is disposed on either one of the upper and the rear surface of one metal plate (unconstrained damping material), products having a sealing function by utilizing the macromolecular visco-elastic layer disposed on the upper surface are widely utilized. However it has a low loss factor and the vibration damping effect thereof is fairly lower than that of the constrained damping material.